The invention relates to a cylinder-piston unit of a needle-free injector, with at least one cylinder accommodating an injection solution, and with at least one piston.
DE 10 2005 054 600 discloses a cylinder-piston unit of a needle-free injector, of which the cylinder and the piston at least temporarily enclose an injection solution. At its front end, the cylinder has at least one outlet nozzle. In the front area, the piston has an elastic skirt, of which the front outer edge, when the piston is unloaded, covers a cross-sectional surface area larger than a cross-sectional surface area of the inner wall of the cylinder.
Pistons of this kind cannot generally be fitted free of bubbles into an already filled cylinder of a cylinder-piston unit. For this purpose, a so-called vacuum applicator machine is generally needed.
Another method by which a stopper is placed at ambient pressure onto the surface level of the already filled injection solution is one in which it is radially compressed by means of a multi-finger gripper and placed onto the liquid level. However, this can only be done with elastic pistons, e.g. made of a rubber mixture, which do not generally seal up to 350*105 Pa and are therefore unsuitable for needle-free injectors.
U.S. Pat. No. 1,521,890 A describes a metallic syringe from medicine, having a piston which manages without a rubber-like sealing means. For this purpose, the piston has the shape of a forwardly open pot, in the bottom of which a central inner thread is arranged. By way of the inner thread, the piston is screwed onto the piston rod such that the latter also protrudes forwards from the pot. The pot wall bearing on the cylinder tapers towards the front. A frustoconical expansion body sits in the hollow space of the pot. It is likewise screwed onto the piston rod. When the expansion body is screwed in, the pot wall is pressed against the cylinder wall.
The object of the present invention is therefore to develop a piston whose components, at ambient pressure, can be set down at least partially on the liquid level of a filled cylinder in a bubble-free, sterile and gas-tight manner and which nevertheless withstands a high compression pressure without leakage during regular emptying of the cylinder-piston unit.